Lis Valley (Portugal)
The Lis Valley Irrigation District is a perimeter of the Centre of Portugal (Figure 1), with an area of 2.000 ha managed by the Lis Valley Water Users Association. Its irrigation and drainage systems date back to 1957. An annual average of approximately 7.1 million m3 of water is used to grow maize, rice, forage, and horticultural crops.
Figure 1: (a) Location of the Lis Valley Irrigation District in Portugal (red square) (source: Google Maps, https://maps.google.pt); (b) Location of rice fields (rose color) in Lis Valley Irrigation District (source: Lis Valley Irrigation District Water Users’ Association), including the location of the Pilot Farms.
The Lis Valley has a Mediterranean temperate climate, with extended and temperate summers and winters of mild temperatures (2020 daily temperatures presented in Figure 2). Precipitation is concentrated in the wetter months (October to March), with an annual average of 800-900 mm. The type of soil is alluvial, with several textures, being the silty-loam the more representative. Land is very flat and regular, with an altitude between 2 and 20 m.
The water supply to the open canal’s conveyance network relies on weirs installed along the Lis river and tributaries. In the dry season, water pumped from drainage ditches is also used. The irrigation network is comprised of 17 weirs, a primary irrigation network of 44.5 km length, and a secondary one of 180 km of small lined or earthen channels. The main problems with water distribution are the water shortage and poor water quality in the summer (dry period), and the absence of automation mechanisms to control the water levels in the channels network.
Rice is cultivated in an area about 140 ha, mainly in lower lands with poor drainage conditions and, in some cases, with soil salinity problems. Wet seeding is the applied technique, and the irrigation is made by continuously flooding method (water levels above soil surface observed in 2019 is presented in Figure 3). Modern technology is applied, namely precise laser levelling, improved seeds, and intensive sustainable agrochemicals. The main constraint in increasing the cultivated area is the water shortage.
The widespread rice agronomic practices are the following: i) Wet seeding after the initial flooding, sowing is made mechanically by centrifugal spread, usually in the beginning of May; ii) the basins are drained before herbicide or pesticide applications; iii) fields are maintained flooded until the last crop phase but are kept dry during at least 2‐3 weeks before harvest, usually in October.
Figure 2: Minimum, average, and maximum daily air temperatures during 2020 rice crop season.
Figure 3: Typical continuous flooding water elevation above soil surface (observed during 2020 rice crop season).
PROBLEMS AND DIFFICULTIES
The main problems faced by rice irrigation in Lis Valley include: i) Limitations in irrigation water availability due to scarcity at source – it is an endemic problem during summer season, which is aggravated due to the weaknesses of the water distribution system; ii) Constraints on the conveyance hydraulic system, including the energy required for water pumping – it is aggravated by several factors, namely the requirement to pump for drainage, and the operation of some age-old pumping stations; iii) Environmental risk related with soil salinization – it occurs in some areas with saline bedrock, or in lowlands due to sea water intrusion.
Rice production has a relatively high potential to increase in the Lis Valley because of the farmers’ know-how about this economically profitable and competitive crop. As water scarcity is the main constraint to increasing the rice production area in this valley, water saving practices for rice irrigation may leverage this process. Alternate wet and dry, or complementary wastewater applications, are examples of welcome practices; however, these practices have not yet been fully tested. On the other hand, the company Águas do Centro Litoral SA (AdCL), which operates a wastewater plant located in the Lis Valley, is ready to provide a significant amount of treated wastewater, a scarce resource during summer season. The adoption of this solution is a critical issue because farmers and technicians have no experience in applying this type of irrigation water, and there are uncertainties and concerns about the inherent risks to food safety, farmers’ health and soil conservation.
The aims of the study are the following: i) Improving rice irrigation water use at field level, focusing on the optimization of water productivity related to upgrading soil and crop management techniques; ii) Testing alternative water saving techniques of rice irrigation at field level; iii) Testing the use of urban treated wastewater in rice irrigation, at laboratory pot and field plot levels, and assessing its yield impacts, and health and ecotoxicological risks; and iv) Improving farmers’ economic outcomes due to the improvement of rice production technologies, and the increase of the rice cultivation area, allowed by the implementation of water saving practices and the use of treated waste water for rice irrigation.
The Lis Valley case study is supported by the establishment of three experimental rice field plots divided between two pilot farms, and a laboratory trial (Figure 1):
The main experimental activities and data measured are the following:
Pilot Farm 1 – Nuno Guilherme Farm (Monte Real) is a private farm that cultivates around 60 ha of rice. The experimental study focusses on the application of traditional flooding irrigation in one rice field pilot plot (plot 1; area of 2.8 ha) and AWD flooding irrigation in another neighbouring pilot plot (plot 2; 0.18 ha). Measured data include irrigation water inflow and outflow, water use efficiency, surface water depths, soil water moisture, yield, and farm profitability.
Pilot Farm 2 – Delfim Eugénio Farm (Lameira, Leiria) is a private horticultural farm, that provided a small plot (240 m2) for the drip irrigation trial. The drip irrigation experiment carried out on a plot of a private farmer that usually produces horticultural crops. The soil has a light texture, very good internal drainage, with deep groundwater level. A water was supplied using an electric pump. The irrigation system comprised filters, fertigation injector, water counter and pressure gauges; PE drip lines of 16.2 mm diameter, placed in the soil surface was applied. The monitoring system installed in this plot evaluated the following aspects: water use, soil moisture, crop development, yield quantity and quality.
Pot Experiment – Rice crop cultivated in pots and irrigated with urban treated wastewater from the AdCL plant (Coimbrão, Leiria), to assess the irrigation of subsurface drip. This experiment investigates the effects of using irrigation treated wastewater, on yield and on soil drainage water. Measured data include irrigation depth, water, and soil quality (microbiology, ecotoxicological and chemical), yield and rice grain quality, namely traces of some heavy metals.